This invention relates to liquid crystal displays and more particularly to such displays having multiple levels.
With the widespread use of silicon chips to digitally control the displays of watches, games, hand-held calculators, and similar devices came the ability to communicate to the operator more information than he normally or routinely demanded. This data or information is generally stored in memory or a register and is not displayed until it is needed by the operator.
Since such devices typically are hand-held, there exists a limited work space or display area available to communicate information. To accomodate such restrictions, same display window is often used to communicate both commonly used data, and, upon selection by the operator, specialized or non-routine data. One way to achieve multiple uses of the same display space is by means of a stacked display, which enables the same display window or area to communicate different types of data.
The stacked display basically consists of two or more levels of electro-optic cells, such as liquid crystal segments. Such cells or segments are normally transparent but become opaque when electrically activated. By applying electrical stimulation to certain segments of a display, different information may be selectively communicated to the operator. A prior-art stacked display is described in U.S. Pat. No. 4,013,901 issued to Clark Williams on Mar. 23, 1977.
Due to the characteristics of liquid crystal displays, an alternating current must be used to maintain reliability while obtaining satisfactory contrast ratios. This requirement of alternating current has led to the development of multiplexing techniques such as the half duty cycle, third duty cycle, and others. Liquid crystal display segments are not responsive to instanteous impulses but are responsive to root mean squared (RMS) potentials. The various multiplexing schemes take advantage of the nonlinear optical response to the applied RMS potential, such that an "off" RMS potential effects a response less than an optical threshold and an "on" potential causes optical saturation of the liquid crystal material.
The presently preferred way of carrying out the invention wil now be described with reference to the accompanying drawings.